Climate and air quality impacts due to mitigation of non-methane near-term climate forcers

Abstract

Abstract. It is important to understand how future environmental policies will impact both climate change and air pollution. Although targeting near-term climate forcers (NTCFs), defined here as aerosols, tropospheric ozone, and precursor gases, should improve air quality, NTCF reductions will also impact climate. Prior assessments of the impact of NTCF mitigation on air quality and climate have been limited. This is related to the idealized nature of some prior studies, simplified treatment of aerosols and chemically reactive gases, as well as a lack of a sufficiently large number of models to quantify model diversity and robust responses. Here, we quantify the 2015–2055 climate and air quality effects of non-methane NTCFs using nine state-of-the-art chemistry–climate model simulations conducted for the Aerosol and Chemistry Model Intercomparison Project (AerChemMIP). Simulations are driven by two future scenarios featuring similar increases in greenhouse gases (GHGs) but with “weak” (SSP3-7.0) versus “strong” (SSP3-7.0-lowNTCF) levels of air quality control measures. As SSP3-7.0 lacks climate policy and has the highest levels of NTCFs, our results (e.g., surface warming) represent an upper bound. Unsurprisingly, we find significant improvements in air quality under NTCF mitigation (strong versus weak air quality controls). Surface fine particulate matter (PM2.5) and ozone (O3) decrease by -2.2±0.32 µg m−3 and -4.6±0.88 ppb, respectively (changes quoted here are for the entire 2015–2055 time period; uncertainty represents the 95 % confidence interval), over global land surfaces, with larger reductions in some regions including south and southeast Asia. Non-methane NTCF mitigation, however, leads to additional climate change due to the removal of aerosol which causes a net warming effect, including global mean surface temperature and precipitation increases of 0.25±0.12 K and 0.03±0.012 mm d−1, respectively. Similarly, increases in extreme weather indices, including the hottest and wettest days, also occur. Regionally, the largest warming and wetting occurs over Asia, including central and north Asia (0.66±0.20 K and 0.03±0.02 mm d−1), south Asia (0.47±0.16 K and 0.17±0.09 mm d−1), and east Asia (0.46±0.20 K and 0.15±0.06 mm d−1). Relatively large warming and wetting of the Arctic also occur at 0.59±0.36 K and 0.04±0.02 mm d−1, respectively. Similar surface warming occurs in model simulations with aerosol-only mitigation, implying weak cooling due to ozone reductions. Our findings suggest that future policies that aggressively target non-methane NTCF reductions will improve air quality but will lead to additional surface warming, particularly in Asia and the Arctic. Policies that address other NTCFs including methane, as well as carbon dioxide emissions, must also be adopted to meet climate mitigation goals.

Historical and future changes in air pollutants from CMIP6 models

“…Smaller reductions in global annual mean surface O 3 of 4 ± 1.7 ppb are predicted for the middle-of-the-road pathway (ssp245) by 2100, whereas for the weak climate and air pollutant mitigation scenario ssp370, a global annual mean increase in surface O 3 of 1.6±0.9 ppb in 2050 and 0.6±1.0 ppb by 2100 is predicted. However, implementing strong emis-sion controls for SLCFs on top of a weak climate mitigation scenario (ssp370-lowNTCF) shows that previous increases in global annual mean surface O 3 can be substantially reduced to values that are 2.5 ± 0.5 ppb below the 2005-2014 mean value in 2050, with benefits to air quality and climate (Allen et al, 2020). For ssp585, which has weak climate mitigation measures but strong air pollution controls, a near-term increase in global annual mean surface O 3 of 1.4 ± 0.8 ppb is predicted in 2050, but by 2100 surface O 3 reduces by 2.7 ± 1.5 ppb, relative to 2005-2014, due to the implementation of air pollutant controls in the latter half of the 21st century.…”

“…However, substantial reductions in PM 2.5 concentrations of 5.6 ± 2.0 µg m −3 and 5.3 ± 2.1 µg m −3 below 2005-2014 values are achieved by 2050 across East and South Asia, respectively, by implementing these measures. Due to the short lifetime of aerosols in the atmosphere, PM 2.5 concentrations respond rapidly to the large cuts in emissions that occur in ssp370-lowNTCF and show the benefits of targeting these emissions, although there could be a potential climate impact (Allen et al, 2020).…”

“…Two major components of air pollution at the surface are ozone (O 3 ) and particulate matter less than 2.5 µm in diameter (PM 2.5 ). Exposure to present-day ambient concentrations of these two air pollutants was estimated to cause up to 4 million premature deaths per year (Apte et al, 2015;Malley et al, 2017). Over recent decades, the impact on human health from exposure to air pollutants has been increasing (Butt et al, 2017;Cohen et al, 2017).…”

A continued role of short-lived climate forcers under the Shared Socioeconomic Pathways

“…SLCFs, and in particular aerosols, also play a key role in shaping local and regional hydrology and dynamics. Comparing the SSP3-7.0 and SSP3-lowNTCF scenarios, Allen et al (2020) recently found a significant precipitation increase due to removal of aerosols, with the strongest moistening trends over Asia. An increase in the Asian summer monsoon precipitation in scenarios with strong air pollution reductions was also recently found by Wilcox et al (2020).…”

Future evolution of aerosols and implications for climate change in the Euro-Mediterranean region using the CNRM-ALADIN63 regional climate model

“…Finally, due to their role as condensation nuclei, aerosols can also modify the microphysical and radiative cloud properties, which notably have consequences on cloud albedo and lifetime (indirect effect; Twomey, 1977;Albrecht, 1989;Lohmann and Feichter, 2005). Over the past few years, efforts have been made to quantify the magnitude of the different aerosol radiative effects on the radiative budget (Boucher et al, 2013;Myhre et al, 2013a;Stevens, 2015;Allen et al, 2020) but their quantification still shows large uncertainties (Myhre et al, 2020;Bellouin et al, 2020).…”